1. Field of the Invention
The present invention relates to an image processing apparatus in an electronic file or a digital copying machine, and more particularly to an image processing apparatus capable of separating and discriminating binary image components such as characters and half-tone image components in one image.
2. Description of the Prior Art
In an office equipment for processing an image in an electronic file or a digital copying machine, two competing demands, one for clearly reproducing boundaries of white and black areas such as for a character image and the other for reproducing half-tone areas while deemphasizing the boundaries of the white and black areas, such as for a photographic image have been requested.
The former demand has been met by improving the image resolution power of the equipment and the latter demand has also been substantially met by a systematic dither method recently developed in the digital image processing field.
However, an original document which a user inserts is to the equipment is not always clearly classified as a character image (binary image) or a half-tone image, and in many cases, character images, graphic images and photographic images exist in one original document.
Since it is difficult to discriminate the binary image and the half-tone image in one original document, the images have been processed by either binary recording technique (in which the image data is binarized by a predetermined threshold to record it as on/off dots) or a half-tone recording technique (in which the image data is binarized by a dither matrix to record it as on/off dots). The binary recording technique which clearly reproduces white and black areas is suitable to reproduce a binary image such as a character image or bar graph image and a band compression encoding of a transmitted signal is easy to attain, but it is difficult to reproduce a half-tone image such as a photograph or a picture. On the other hand, the dither method can provide a high quality of image for the half-tone image but has a low resolution power. Therefore, the quality of the image is poor for a binary image such as the character image. Accordingly, when the original document contains both the character image and the half-tone image, it is not possible to reproduce the original image with a high quality by only the dither method.
As an approach to solving the above problem, it may be possible to use the systematic dither method for the half-tone image area in the original document and use the binary processing method for the binary image area, such as the character or pattern image of the same document to obtain a high quality of image reproduction.
However, since this method requires discrimination of the binary image area and the half-tone image area, an image memory which stores pixels read from the original document in one-to-one correspondence is required. As the result, as the size of an image screen, pixel density and the number of gray levels to be processed increase, the capacity of the memory increases and a complex circuit is required. As the result, the cost of the memory increases, the reliability of the overall equipment decreases, and the complexity of the memory operation increases.